How to check a datastore for a string?

Alright, so I am trying to make a police database to check warrants and such on a car. I started with code getting everything set up but I got stuck on figuring out how to get a plate number to a new person that wasn't registered already. How would you check the datastore to make sure that a certain math is not their? Heres the code I got so far, please and thanks :)

local datastore = game:GetService("DataStoreService"):GetDataStore("LisencePlatesStorage")
local LisencePlate = "lisencePLATE"

game.Players.PlayerAdded:Connect(function(plr)
    local ID = LisencePlate.."-"..plr.UserId
    local savedData = nil

    local PlateNumber = Instance.new("StringValue")
    PlateNumber.Name = "PlateNumber"
    PlateNumber.Parent = plr

    pcall(function()
        savedData = datastore:GetAsync(ID)
    end)

    if savedData ~= nil then
        PlateNumber.Value = savedData
    else
        --math.random(removed because not decided on yet) 
        --area where stuck on
    end
end)